1. Field of the Invention
This invention relates to modulators for modulating electrical signals, and more particularly to a method and apparatus for calibrating the modulation sensitivity of a modulator by providing a feedback path between a transmitter and receiver and adjusting the modulation sensitivity in response to the transmitter output as detected by the receiver.
2. Description of Related Art
Encoding, or "modulating", information on an electrical signal is an important part of the communication arts. Whether signals are to be transmitted over the air or through a conduit (such as wire or fiber optic cable), the particular manner in which information is modulated impacts the cost and reliability of the system, and is critical in determining the amount of, and the speed at which, such information can be transmitted. Because of limitations in the frequency spectrum that is available for use in transmitting information, it is typically desirable to use as little of the frequency spectrum as possible to transmit information (i.e., increase system bandwidth efficiency). In addition to the desire to use the frequency spectrum efficiently, some systems also require a constant envelope modulation. Constant envelope modulation means that the instantaneous signal power does not fluctuate. Constant envelope modulation is desirable because some devices through which the signal must be processed, such as power amplifiers, have highly nonlinear characteristics which, when passing a signal with power fluctuations, cause distortions of the signal which produce extraneous sidebands. These distortions are normally known as AM-to-PM conversion and intermodulation distortion. Generation of such sidebands defuse the power of the information signals (i.e., reduce the amount of power in the band of interest), and also can interfere with nearby channels (adjacent channel interference) or with other communications systems (co-channel interference). Therefore, it is desirable to provide a modulation technique that maintains an essentially constant amplitude envelope.
Modulation techniques, such as Minimum Shift Keying (MSK), strive to minimize the required bandwidth and have constant envelope modulation. This can be understood as follows. Offset quadrature phase shift keying (OQPSK) is a variation on quadrature phase shift keying (QPSK). FIGS. 1a-1c illustrate the way in which information is modulated in accordance with QPSK. Each bit of information is transmitted over a period T. Accordingly, a data stream d.sub.k (t) is defined which has a value associated with each bit of information at each time t. In the case shown in FIG. 1a, the value of d.sub.k (t) is either +1 or -1. The data signal is divided into an in-phase and quadrature data stream, d.sub.I (t) and d.sub.Q (t). The in-phase data stream d.sub.I (t) comprises all of the even numbered data bits d.sub.0, d.sub.2, d.sub.4, etc., as shown in FIG. 1b and the quadrature data stream comprises all of the odd numbered data bits, d.sub.1, d.sub.3, d.sub.5, etc., as shown in FIG. 1c. Referring to FIG. 2, the in-phase data stream d.sub.I (t) is applied to a first mixer 201 within a modulator 200, and the quadrature data stream d.sub.Q (t) is applied to a second mixer 202. The mixers 201, 202 modulate an in-phase carrier signal of the 1/2 form 1/(2).sup.1/2 cos (.omega..sub.0 t+.pi./4) and a quadrature carrier signal of the form 1/(2).sup.1/2 sin (.omega..sub.0 t+.pi./4) with the data signals d.sub.I (t), d.sub.Q (t) and a summing circuit 203 sums the output from the two mixers 201, 202.
In accordance with OQPSK, the data stream is similarly divided into an in-phase data stream and a quadrature data stream. However, the in-phase and quadrature data streams are offset by one period T as shown in FIG. 1d. Since each data bit within the in-phase and quadrature data streams are maintained for a period of 2T, the in-phase and quadrature signals will never transition at the same time. Accordingly, the resulting sum of the in-phase and quadrature signals will never have more than a 90 degree phase shift at any one time. Therefore, when an OQPSK signal undergoes bandlimiting, the resulting intersymbol interference causes the envelope to droop slightly in the region of the +/-90 degrees phase transition, but since the phase transitions of 180 degrees have been avoided in OQPSK, the envelope will not go to zero as it does with QPSK. When the bandlimited OQPSK goes through a nonlinear transponder, the envelope droop is removed; however, the high-frequency components associated with the collapse of the envelope are not reinforced. Thus, out-of-band interference is avoided.
In accordance with MSK modulation, the problem of phase transitions is avoided completely by modulating a OQPSK signal with a sinusoid having a period of .pi.t/2T, as shown in FIGS. 3(a) and 3(c). This results in the signal shown in FIGS. 3(b) and 3(d). The sum is a signal that has a constant amplitude envelope as shown in FIG. 3(e). Accordingly, MSK modulation can be considered a variation on frequency shift keying (FSK) modulation in which information is encoded by altering the frequency of the carrier between a first and second frequency with a modulation index of 0.5.
However, a problem arises due to the way in which most modulators modulate MSK signals. That is, signals are modulated by applying a baseband information signal directly to a voltage controlled oscillator (VCO). The applied voltage alters the output frequency of the VCO (i.e., the carrier) by an amount that is directly proportional to the applied voltage. In accordance with MSK modulation, the modulation index must be very precisely controlled at 0.5 in order to achieve a deviation frequency of .function..sub.0 +/-1/4T. Any variation from this deviation frequency will distort the orthogonal relationship between the in-phase and quadrature signals. Accordingly, the modulation sensitivity (i.e., the amount of deviation in frequency which occurs as a result of the application of a signal of a particular voltage level) is critical.
This creates a practical problem, since VCO modulation sensitivity tends to vary significantly from unit to unit and over temperature. This variation of the VCO modulation sensitivity results in a variation in the modulation index. Thus, if precise control of the modulation index is required, implementation difficulties arise.
Typically, modulators are calibrated during alignment of the transmitter. That is, once the transmitter has been assembled, a known voltage is applied to the VCO within the modulator.
The resulting deviation from the carrier frequency is then adjusted to ensure that the input signal results in the proper frequency deviation. One method for adjusting the deviation of the modulator is to apply the baseband signal to a signal attenuator (or amplifier) that has an adjustable loss (or gain). The loss (or gain) is adjusted to cause a baseband signal of known amplitude to deviate the carrier by the desired amount. Nonetheless, because the modulation sensitivity can change after the transmitter has been aligned, the modulation index can vary during operation and may be more or less than the required 0.5 modulation index.
Accordingly, there is currently a need for a method and apparatus that allows a modulator within a transmitter to be calibrated during the operation of the transmitter to ensure that the modulation index of a VCO remains calibrated during the operation of the modulator regardless of variations in the temperature of the modulator or other modulator operating conditions.